Formed articles obtained by thermoforming resin sheets are widely used in various applications, such as food containers, automotive parts, building materials, and home appliance parts. In particular, formed articles containing polypropylene resins as base materials are suitable for applications requiring their high heat resistance, high chemical resistance or other properties.
However, unfortunately, polypropylene resin sheets greatly sag during preheating in thermoforming (this sagging phenomenon is hereinafter referred to as “drawdown”), thereby likely causing defects such as formation of wrinkles in formed products. For this reason, there are restrictions on forming conditions, such as that these sheets are unable to be sufficiently heated. Particularly, in order to meet the increasing demand for deep draw forming, the sheets have been required to show less drawdown than the prior art and excellent elongation during shaping.
In order to solve these problems, for example, Patent Literature 1 discloses a sheet formed from a composition containing a specific polypropylene together with a specific low-density polyethylene, a high-density polyethylene, and talc. However, in this method, a relatively large amount of the polyethylene resins are added to the polypropylene resin, which disadvantageously leads to impairment of the heat resistance of the polypropylene resin and deterioration of the appearance.
In addition, for example, Patent Literature 2 discloses a sheet formed from a composition containing a high molecular weight polyethylene with a specific intrinsic viscosity and a polypropylene resin having a specific melt tension. This method, however, does not provide a sufficient drawdown-reducing effect.
Furthermore, for example, Patent Literature 3 discloses a polypropylene resin for a sheet having a specific MFR and a specific melt tension, obtained by melt kneading a propylene polymer and a peroxydicarbonate. However, the resin obtained by this method also does not have a sufficient drawdown-reducing effect and does not show sufficient elongation during shaping. Therefore, unfortunately, the resulting sheet may break when it is stretched.